Documentation of the pi_state refcounting in the requeue code is non
existent. Add it.
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
kernel/futex.c | 36 +++++++++++++++++++++++++++++++++---
1 file changed, 33 insertions(+), 3 deletions(-)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1709,6 +1709,8 @@ static int futex_requeue(u32 __user *uad
* exist yet, look it up one more time to ensure we have a
* reference to it. If the lock was taken, ret contains the
* vpid of the top waiter task.
+ * If the lock was not taken, we have pi_state and an initial
+ * refcount on it. In case of an error we have nothing.
*/
if (ret > 0) {
WARN_ON(pi_state);
@@ -1724,12 +1726,16 @@ static int futex_requeue(u32 __user *uad
* it. So we rather use the known value than
* rereading and handing potential crap to
* lookup_pi_state.
+ *
+ * If that call succeeds then we have pi_state
+ * and an initial refcount on it.
*/
ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
}
switch (ret) {
case 0:
+ /* We hold a reference on the pi state. */
break;
case -EFAULT:
put_pi_state(pi_state);
@@ -1804,19 +1810,38 @@ static int futex_requeue(u32 __user *uad
* of requeue_pi if we couldn't acquire the lock atomically.
*/
if (requeue_pi) {
- /* Prepare the waiter to take the rt_mutex. */
+ /*
+ * Prepare the waiter to take the rt_mutex. Take a
+ * refcount on the pi_state and store the pointer in
+ * the futex_q object of the waiter.
+ */
atomic_inc(&pi_state->refcount);
this->pi_state = pi_state;
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
this->task);
if (ret == 1) {
- /* We got the lock. */
+ /*
+ * We got the lock. We do neither drop
+ * the refcount on pi_state nor clear
+ * this->pi_state because the waiter
+ * needs the pi_state for cleaning up
+ * the user space value. It will drop
+ * the refcount after doing so.
+ */
requeue_pi_wake_futex(this, &key2, hb2);
drop_count++;
continue;
} else if (ret) {
- /* -EDEADLK */
+ /*
+ * rt_mutex_start_proxy_lock()
+ * detected a potential deadlock when
+ * we tried to queue that waiter. Drop
+ * the pi_state reference which we
+ * took above and remove the pointer
+ * to the state from the waiters
+ * futex_q object.
+ */
this->pi_state = NULL;
put_pi_state(pi_state);
goto out_unlock;
@@ -1827,6 +1852,11 @@ static int futex_requeue(u32 __user *uad
}
out_unlock:
+ /*
+ * We took an extra initial reference to the pi_state either
+ * in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
+ * need to drop it here again.
+ */
put_pi_state(pi_state);
double_unlock_hb(hb1, hb2);
wake_up_q(&wake_q);
--
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